This invention relates to the production of multiple-layer sheeting, and more especially, to the extrusion of polystyrene sheeting having a thin protective layer of ABS polymer.
Co-extrusion processes for the production of multilayer sheeting are of course well known in the art; however, hitherto known processes almost exclusively employ some form of encapsulation technique wherein one stream of thermoplastic material, typically the volumetrically smaller stream, is completely surrounded, e.g., coaxially, by a second stream of such material prior to passing the entire composite stream through an extrusion die. Alternatively, the foregoing encapsulation may be effected in the cavity-portion of an extrusion die. In both types of process, the resultant sheeting product is characterized by an inner-layer of one type of thermoplastic material sandwiched between or encapsulated by two exterior layers of a second thermoplastic material.
Very little success, with one possible exception, has been recorded in the area of co-extrusion of multiple thermoplastic resinous layers merely laminated on top of one another. Most attempts have employed a series of extrusion dies whereby two distinct sheets of different material are first formed by extrusion, superimposed upon one another and then passed through yet another extrusion die or other restrictive orifice. In other attempts, there are employed complex extrusion dies wherein individual molten streams of thermoplastic material are superimposed upon one another in the die cavity immediately before passing through the die lips.
The possible exception referred to hereinabove relates to U.S. Pat. No. 3,476,627 which discloses a process comprising combining two streams of molten thermoplastic resin in a conduit upstream of an extrusion die such that they have a sharply defined juncture plane, and thereafter passing the composite stream through an extrusion die in such a manner that the juncture plane is parallel to the principle direction of flow of the resin as it takes on the shape of sheeting. The foregoing disclosure is concerned primarily with the co-extrusion of multiple-layered polyvinyl butyral resin. This patent does, however, refer broadly to a process for co-extruding multiple-layered sheeting of different thermoplastic resins having similar processing characteristics and which will adhere to one another, and further limited by the requirement that the volume of a given resin component be at least 25% of the total resin content.
In contrast, this invention is concerned with the production of multiple layered sheeting of two materials which have different processing characteristics, which are highly incompatible and which have hitherto defied attempts both through co-extrusion techniques and conventional laminating methods to produce a strongly adhering bond therebetween. Prior attempts aimed at co-extrusion of such materials have resulted in multilayered products wherein the individual layers are rather easily peeled from one another, and like attempts to laminate materials of this nature by extruding a molten layer of one polymer onto a preformed sheet of a second polymer have provided similar unsatisfactory results. Moreover, the invention relates to multilayered sheeting consisting primarily of a less expensive material, i.e., polystyrene, and having only a very thin surface layer or veneer of a second, more expensive polymeric material having highly desirable surface characteristics, i.e., ABS polymer.
In grandparent application Ser. No. 368,834, there is described a process for overcoming the problems of prior art and producing a suitable polystyrene/ABS multiple layered sheeting product, however, it has been discovered that not even all ABS polymers may be successfully co-extruded in accordance with the there-disclosed process to provide a multi-layered product wherein sufficient bond strength was exhibited between polystyrene and ABS layers. Even ABS polymers having similar processing characteristics, such as melt index, were unpredictable insofar as the production of bond strength to polystyrene is concerned. There was known no means by which to predict whether or not a given ABS would sufficiently bond to the polystyrene under co-extrusion conditions. In parent co-pending application Ser. No. 519,682, a definition was presented for ABS polymers which could be successfully bonded to a polystyrene substrate by co-extrusion techniques; however, because the phenomenon is so unpredictable, further refinement is required in this definition to prevent the possibility that it unintentionally encompass isolated ABS compositions which do not satisfactorily bond to polystyrene in the claimed process.